Occupancy of the T cell receptor (TCR) is a potent means of inducing programmed cell death (PCD) in thymocytes. Similarly, glucocorticoids induce thymocyte PCD. It has been demonstrated that glucocorticoids antagonize induction of TCR-mediated PCD in T cell hybridomas and thymocytes in vitro. While exposure to either stimulus alone induces PCD, simultaneous stimulation with both allows the cells to survive (mutual antagonism) This finding led to the proposal that exposure of thymocytes to both of these stimuli during thymocyte development may be a basis for the selective maturation of non-autoreactive thymocytes called positive selection. Whereas high avidity TCR/ligand interactions may induce a signal too potent to be antagonized by glucocorticoids resulting in negative selection of self reactive T cells, low-moderate avidity TCR/ligand interactions may transduce a weaker signal that can be antagonized by glucocorticoid/GR interactions allowing the cell to survive and positive selection to occur. For this to be a viable model, there necessarily must be exposure of thymocytes to glucocorticoids during T cell development in the thymus. Because the thymus has been reported to have endocrine properties, we investigated whether the thymus can synthesize steroids. Immunohistochemical analysis of the thymus revealed that steroidogenic enzymes are present in a subset of thymic epithelial cells resulting in synthesis of detectable levels of steroids in vitro. Experiments were next performed to evaluate the role of thymic-synthesized steroids during thymocyte development. The effects of deprivation of steroids by steroid synthesis inhibitors during fetal thymic organ cultures (FTOC) were analyzed for effects on positive selection, as assayed by development of thymocytes to a mature phenotype. These experiments were performed with mice that express a transgenic TCR specific for the H-Y antigen expressed in the context of MHC H-2b. In H- 2b female mice, transgenic TCR-expressing T cells are positively selected, whereas in transgenic TCR mice that express H-2k, a non- selecting MHC phenotype, no positive selection occurs. In FTOC with thymuses from TCR-transgenic H-2b female mice inclusion of steroid synthesis inhibitors resulted in severe depletion of thymocytes, whereas in TCR-transgenic H-2k female mice there is almost no loss of thymocytes. These results imply that thymic synthesized steroids play a role in thymocyte development, specifically during positive selection by antagonizing TCR-mediated death signals. Further experiments are being performed to address the effects and specificity of steroids during thymocyte development in normal mouse thymus.